This invention relates to surface acoustic wave devices and, more particularly, to improved transducers for surface acoustic wave devices.
In recent years, surface acoustic wave (SAW) devices have been employed in an increasing number of practical applications such as filters and delay lines. In a typical SAW device, opposing "combs" of interdigitated metal fingers are disposed on a piezoelectric material and a surface acoustic wave can be induced in the material by electrically exciting the fingers or, conversely, an electrical signal can be induced across the fingers by a surface acoustic wave propagating in the piezoelectric material beneath the fingers. The geometrical configuration of the fingers is selected to yield desired electrical characteristics; for example, the spacing between fingers often being selected as a function of the primary wavelength to be propagated in the device. A technique known as "tap-weighting" is presently in widespread use, e.g. where the SAW is to be used for a bandpass, chirp or matched filter. In the most common form of tap-weighting, known as "apodization", the metal fingers of each comb vary in length by a factor of as much as 50. In other words, each comb may have many fingers of different lengths with the longest finger being of the order of 50 times the length of the shortest finger. This wide diversity in finger lengths, when properly designed, yields desirable characteristics, such as an effective frequency bandpass. However, tap-weighting techniques give rise to practical problems which limit their usefulness. One problem manifests itself in the size (and therefore, cost) of the SAW device using tap-weighted finger lengths. Specifically, since the shortest finger employed must have at least a certain minimum required length (as dictated to some degree by its width which is wavelength dependent and its minimum length with respect to width), it follows that the longest finger takes on a size which necessitates a relatively large overall device size. In the fabrication of SAW devices, manufacturing costs relate to device size, and the necessarily larger devices are therefore more expensive. A further problem with tap-weighting is that the amplitude profile of the generated surface acoustic wave, which obtains differing contributions from fingers of different lengths, is non-uniform.
It has been suggested that some of the above-identified problems could be solved by achieving tap-weighting by using a capacitor in series with the different fingers of each comb so that finger lengths would not have to be varied. In particular, the tap-weighting would be attained by employing different sized capacitors in series with the various fingers of the comb so as to get the desired weightings by virtue of the differing inpedances presented to the particular operating signal frequencies. However, this technique gives rise to a fundamental limitation in that the strength of a transducer tap cannot be varied without varying the tap delay.
A further existing problem of SAW devices is a result of the bi-directionality of conventional interdigitated transducers. In particular, when a transducer is excited with the intention of producing a surface acoustic wave propagating in a particular direction, a spurious surface acoustic wave propagating in the opposite direction is also generated. This results in signal loss and the generation of spurious signals, the reflection of which causes undesired interference. So-called "unidirectional" transducers have been devised and typically have electrodes consisting of finger triplets (rather than the finger pairs of conventional bi-directional transducer combs). The electrical excitation to the three fingers is appropriately phased such that the wave components add constructively in the desired propagation direction and interfere destructively in the opposite direction so that effective "unidirectional" operation is achieved. Generally, the finger triplets are components of three different combs with one of the combs geometrically opposing the other two combs. A serious problem arises on the side having the two combs since there is necessarily an overlap of conductors on this side. The result is an undesirable inter-conductor capacitance which causes an unbalanced condition and adversely affects the frequency response of the device.
It is an object of the present invention to provide an improved SAW transducer which, inter alia, is responsive to existing problems of the prior art and, in particular, provides solutions to the problems encountered in tap-weighting techniques and techniques for obtaining unidirectional operation.